682 PHILOSOPHICAL TRANSACTIONS. [aNNO 16S3, 



draw the said line on the globe by the western edge of the horizon, it passes 

 over all those places where the sun is then rising. 



Jupiter being in consequence of the sun, add the difference of his and the 

 sun's right ascensions to the said meridional longitude, bring the degree of the 

 equator answering to their sum under the meridian ; raise the pole next Jupiter 

 equal to his declination^ and detaining the globe in this position, draw a line 

 again by the eastern horizon, the space intercepted between this and the line of 

 the sun's settings, before described on the globe, comprehends all those places 

 on the earth from sun setting till 1/. is set. But if 1/. were in antecedence of 

 the sun, subtract the difference of his and the sun's right ascensions from the 

 meridional longitude, set the degree of the equator answering to the remainder 

 under the meridian, and elevate the pole next Jupiter equal to his declination ; 

 keeping the globe in this position, draw a line by the western edge of the horizon, 

 then the space included between this and the line of the sun's risings, contains all 

 those places on the earth where this eclipse is visible between ^'srisingand sunrise. 



When any eclipse of these is observed, the difference between the noted 

 time and that in the catalogue, will be the difference of meridians between the 

 place of the observation and London, which lies so near the meridian of the 

 observatory, that the distance need not be accounted for. And this determina- 

 tion may be relied on, if the first or third satellite were observed ; but I dare 

 not be so confident of the second and fourth for the reasons formerly given. 

 However, I shall make it my business to observe all such eclipses of as many of 

 them as shall be visible with us, that by comparing my observations with such 

 as shall be made abroad, the error, if any, may be discovered and corrected. 



When 1/. is in quartile of the sun, the distance of the first satellite from his 

 next limb, when it falls into his shadow and is eclipsed, is one semidiameter of 

 ly: ; of the second, 2, or a whole diameter nearly; of the third, 3; of the 

 fourth, 5 of his semidiameters, or something better when the parallax of the 

 orb is greatest. But these quantities diminish gradually as he approaches the ^5 

 or § of the sun, somewhat nearly, but not exactly, in the proportion of the signs. 



As the sun removes from the c5 of ^ , the ingresses of the satellites into his 

 shadow become observable. When he is about 30° from it, the emersions of 

 the 4th, and at 6o°, of the 3d, begin to be seen between the shadow and body; 

 continuing so till the sun be arrived within 6o° of the <? of "]/ , when the emer- 

 sions of the 3d fall behind his body, but the emersions of the 4th continue visible 

 till he be less than 30° distant from the § , at which time they also are hid be- 

 hind him, all the appearances being made really to the right hand or in antece- 

 dence of %, though with inverting telescopes they appear the contrary, to the left. 



After the opposition of the O and 1/. , we begin to see the emersions of all 

 the satellites from the shadow, now on the left hand or in consequence of 1/., 



